A polyolefin which has an excessively high content of heptane-solubles may have a tendency to stick and is therefore difficult to convey and, as a result, is not very suitable for industrial applications. In addition, in the alimentary field, the presence of solubles in a polyolefin which is intended to come into contact with foodstuffs is deemed to be undesirable. For these reasons, for example, isotactic polypropylene preferably has a heptane-insolubles content (denoted by HI, from the expression “heptane-insoluble”) higher than 80% by weight.
Patent Application EP 250229 teaches that the use of certain silanes during the polymerization of olefins allows the hexane-solubles content of the polyolefin obtained to be reduced.
The paper by R. West, Journal of the American Chemical Society (1954) 76, 6012, describes a method for the preparation of 1,1-dimethoxysilacyclohexane. This preparation involves numerous stages and the intermediate formation of a chlorosilacycloalkane which is particularly tricky to handle and easily degradable.
The process of the present invention is particularly simple, involves raw materials which are easily available and relatively stable and does not involve any chlorosilacycloalkane. The stability of the materials used reduces the risk of side reactions, thereby tending in the direction of better purity of the products which are finally prepared.
The presence of alkoxysilacycloalkanes in the environment for the polymerization or copolymerization of at least one olefin is reflected in an appreciable increase in the polyolefin yield and in an appreciable increase in the HI of the said polyolefin in addition, the alkoxysilacycloalkane acts as a morphology protector in the suspension and gas-phase polymerization or copolymerization processes. This means that, in the case of these so-called heterogeneous processes, the polymer or copolymer formed is a better morphological replica of the initial solid catalytic component if an alkoxysilacycloalkane is introduced as an external electron-donor into the polymerization or copolymerization environment.
The process according to the invention includes the stage of reaction between an alkylenedimagnesium dibromide of formula Br—Mg-A-Mg—Br in which A is a divalent alkylene radical optionally substituted, for example by an alkyl radical containing, for example, from 1 to 6 carbon atoms, the said alkylene radical containing from 4 to 7 carbon atoms, the optional substituent(s) being excluded, and a tetraalkoxysilane of formula (OR1)(OR2)(OR3)(OR4)Si in which R1, R2, R3 and R4, which may be identical or different, denote linear or branched, saturated and/or unsaturated hydrocarbon radicals which may include a ring.
The radicals R1, R2, R3 and R4 preferably are alkyl radicals containing from 1 to 6 carbon atoms.
The reaction may be carried out in a solvent which preferably exhibits a Lewis base character, as is the case with ethers. The solvent may, for example, be diethyl ether.
The quantity of inert solvent which is employed may, for example, be such that, assuming the reaction yield to be equal to 100%, the alkoxysilacycloalkane formed is encountered again in a concentration of between 0.05 and 2 moles/liter.
The reaction may be carried out, for example, between 0 and 50° C. for 10 min to 12 hours, if appropriate under pressure if the volatility of the species used makes this necessary, bearing in mind the temperature chosen. Since the reaction is generally exothermic, it is preferable to bring the dibromide and the tetraalkoxysilane into contact gradually and with stirring so as to retain control of the temperature of the mixture. The reaction results in the formation of at least one alkoxysilacycloalkane of formula
in which X and Y denote groups forming part of the group of the radicals R1, R2, R3 and R4 and in which A retains the meaning given above. The ring of the alkoxysilacycloalkane therefore contains a silicon atom and a number of carbon atoms equal to the number of carbon atoms which the alkylene radical A contained, the optional substituents of the said alkylene radical being excluded.
The alkoxysilacycloalkanes in the case of which A is an alkylene radical containing at least one alkyl substituent are also a subject-matter of the present invention.
By way of example, Table 1 below mentions some alkoxysilacycloalkanes which can be prepared by the process according to the invention, by reaction of tetramethoxysilane with an alkylenedimagnesium dibromide, depending on the nature of the divalent alkylene radical A included in the alkylenedimagnesium dibromide.
TABLE 1Nature of AAlkoxysilacycloalkane formedtetramethylene1,1-dimethoxysilacyclopentane1-methyltetramethylene1,1-dimethoxy-2-methylsilacyclopentane1-ethyltetramethylene1,1-dimethoxy-2-ethylsilacyclopentane1-n-propyltetramethylene1,1-dimethoxy-2-n-propylsilacyclopentane1-isopropyltetramethylene1,1-dimethoxy-2-isopropylsilacyclopentane1-n-butyltetramethylene1,1-dimethoxy-2-n-butylsilacyclopentanepentamethylene1,1-dimethoxysilacyclohexane1-methylpentamethylene1,1-dimethoxy-2-methylsilacyclohexane1-ethylpentamethylene1,1-dimethoxy-2-ethylsilacyclohexane1-n-propylpentamethylene1,1-dimethoxy-2-n-propylsilacyclohexane1-isopropylpentamethylene1,1-dimethoxy-2-isopropylsilacyclohexane1-n-butylpentamethylene1,1-dimethoxy-2-n-butylsilacyclohexane2,3-dimethyltetramethylene1,1-dimethoxy-3,4-dimethylsilacyclopentane1,4-dimethyltetramethylene1,1-dimethoxy-2,5-dimethylsilacyclopentanehexamethylene1,1-dimethoxysilacycloheptane
The reaction also gives rise to the formation of BrMgOZ in which Z is a radical forming part of the group of the radicals R1, R2, R3 and R4. This BrMgOZ, considered as being a by-product in the context of the present invention, is generally solid and can, in this case, be removed, for example by filtration. After evaporation of the optional solvent employed and of any excess reactants, the alkoxysilacycloalkane may be purified by distillation, preferably at reduced pressure, for example between 1 and 1×103 mbar.
The alkylenedimagnesium dibromide of formula Br—Mg-A-Mg—Br may be prepared, for example, by reaction between a dibromoalkane of formula Br-A-Br and magnesium in the presence of a solvent, for example an ether like diethyl ether, for example between 0 and 50° C., if appropriate under pressure if the volatility of the species used demands this, bearing in mind the temperature chosen.
The alkoxysilacycloalkanes capable of being obtained by the process according to the invention may be used as an electron-donor in the polymerization or copolymerization of at least one olefin. For example, the silacycloalkane may be introduced within a solid catalytic component of the Ziegler-Natta type and may act as an internal electron-donor.
It is also possible to employ it as an external electron-donor in an environment for the polymerization or copolymerization of at least one olefin, so as to reduce the hexane-solubles content of the polymer or copolymer finally prepared.
In the case of this latter application (external electron-donor) it is preferred to employ an alkoxysilacycloalkane of formula (I) in which X and Y denote methyl radicals.
The alkoxysilacycloalkane preferably contains at least one alkyl substituent positioned alpha to the silicon atom. Particularly high HI values are obtained when the alkyl substituent contains at least two carbon atoms. An excellent compromise of properties (very high HI and generally high yield) is obtained when the alkyl substituent contains 2 or 3 carbon atoms, as is the case with 1,1-dimethoxy-2-ethylsilacyclopentane, 1,1-dimethoxy-2-n-propylsilacyclopentane, 1,1-dimethoxy-2-isopropylsilacyclopentane, 1,1-dimethoxy-2-ethylsilacyclohexane, 1,1-dimethoxy-2-n-propylsilacyclohexane and 1,1-dimethoxy-2-isopropylsilacyclohexane.
In accordance with a specific aspect of the present invention there is provided a process for polymerizing at least one olefin in the presence of a catalyst and of a dialkoxysilacyclohexane of the formula
in which R1 and R2, which may be identical or different, represent alkyl radicals containing 1 to 5 carbon atoms, and X and Y, which may be identical or different, are alkyl radicals containing 1 to 6 carbon atoms. R1, R2, X and Y may be selected from the radicals methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl and isobutyl. Preferably, at least one radical among R1 and R2 is an ethyl radical. More preferably still, R1 and R2 are both ethyl radicals.
The Applicant has found that the presence of the dialkoxysilacyclohexane of formula (1) in the polymerization medium resulted in a polymer having a higher heptane insolubility index coupled with, in general, a greater productivity, in comparison with an identical process in which the dialkoxysilacyclohexane was replaced by the same number of moles of a dialkoxysilacyclohexane of identical structure but in which at least one of the radicals R1 and R2 was replaced by a hydrogen atom.
The following dialkoxysilacyclohexanes may be used:    1,1-dimethoxy-2,6-dimethylsilacyclohexane,    1,1-dimethoxy-2,6-diethylsilacyclohexane,    1,1-dimethoxy-2,6-di-n-propylsilacyclohexane,    1,1-dimethoxy-2,6-diisopropylsilacyclohexane,    1,1-dimethoxy-2,6-di-n-butylsilacyclohexane,    1,1-dimethoxy-2-ethyl-6-methylsilacyclohexane,    1,1-dimethoxy-2-ethyl-6-n-propylsilacyclohexane,    1,1-dimethoxy-2-ethyl-6-isopropylsilacyclohexane,    1,1-dimethoxy-2-n-butyl-6-ethylsilacyclohexane,    1,1-diethoxy-2,6-dimethylsilacyclohexane,    1,1-diethoxy-2,6-diethylsilacyclohexane,    1,1-diethoxy-2,6-di-n-propylsilacyclohexane,    1,1-diethoxy-2,6-diisopropylsilacyclohexane,    1,1-diethoxy-2,6-di-n-butylsilacyclohexane,    1,1-diethoxy-2-ethyl-6-methylsilacyclohexane,    1,1-diethoxy-2-ethyl-6-n-propylsilacyclohexane,    1,1-diethoxy-2-ethyl-6-isopropylsilacyclohexane, and    1,1-diethoxy-2-n-butyl-6-ethylsilacyclohexane.
1,1-Dimethoxy-2,6-diethylsilacyclohexane is particularly preferred.
The dialkoxysilacyclohexane acts as an electron donor during the polymerization. When the catalyst comprises a solid catalytic component, of the Ziegler-Natta type, for example, the dialkoxysilacyclohexane may be incorporated inside the said solid component and be able to act as an internal electron donor.
It is also possible to have the dialkoxysilacyclohexane act as an external electron donor in order to reduce the proportion of heptane solubles in the polymer finally prepared. In this latter case, the dialkoxysilacyclohexane may be introduced into the polymerization medium independently of the solid catalytic component. When the catalyst system involves a solid catalytic component and a cocatalyst, it is also possible to effect precontact between the solid catalytic component, the catalyst and the dialkoxysilacyclohexane before entry into the polymerization medium.
The alkoxysilacycloalkane is generally introduced in a proportion of 1×10−4 to 0.2 millimoles per mole of olefin to be polymerized or copolymerized. If the alkoxysilacycloalkane has been prepared in the presence of a solvent of basic character in the Lewis sense, it is recommended to remove the latter before the polymerization or copolymerization stage because it may have an undesirable influence on the structure of the polymers formed. On the other hand, the alkoxysilacycloalkane may be introduced in the presence, for example, of an aliphatic, alicyclic or aromatic hydrocarbon solvent which is not obviously of a basic nature in the Lewis sense, like hexane, cyclohexane or toluene.
A solid catalytic component containing a transition metal is generally introduced into the polymerization or copolymerization environment.
The transition metal may be chosen from the elements of groups 3b, 4b, 5b, 6b, 7b and 8, lanthanides and actinides, of the Periodic Classification of the elements, as defined in the Handbook of Chemistry and Physics, sixty-first edition, 1980–1981. These transition metals are preferably chosen from titanium, vanadium, hafnium, zirconium and chromium.
The solid catalytic component may be of the Ziegler-Natta type and may, for example, be in the form of a complex containing at least the elements Mg, Ti and Cl, the titanium being in the TiIV and/or TiIII chlorinated form. The solid component may include an electron-donor or acceptor.
A catalytic component of the Ziegler-Natta type is usually the result of the combination of at least one titanium compound, a compound of magnesium and chlorine and optionally an aluminium compound and/or an electron-donor or acceptor, as well as any other compound that can be employed in a component of this type.
The titanium compound is usually chosen from the titanium chlorine compounds of formula Ti—(OR′)xCl4-x in which R′ denotes an aliphatic or aromatic hydrocarbon radical containing from one to fourteen carbon atoms or denotes COR5 with R5 denoting an aliphatic or aromatic hydrocarbon radical containing from one to fourteen carbon atoms, and x denotes an integer ranging from 0 to 3.
The magnesium compound is usually chosen from the compounds of formula Mg(OR6)nCl2-n, in which R6 denotes hydrogen or a linear or cyclic hydrocarbon radical and n denotes an integer ranging from 0 to 2.
The chlorine present in the component of Ziegler-Natta type may originate directly from the titanium halide and/or the magnesium halide. It may also originate from an independent chlorinating agent such as hydrochloric acid or an organic halide like butyl chloride.
Depending on the nature of the transition metal included in the solid-catalytic component, it may be necessary to add to the polymerization environment a cocatalyst capable of activating the transition metal of the solid component. The catalyst of the polymerization process according to the invention may therefore be a multi-component catalyst system, such as the combination of a solid catalytic component and a cocatalyst. If the transition metal is titanium, the cocatalyst may be chosen from organic aluminium derivatives.
This organic aluminium derivative may be a derivative of formula R7R8R9Al in which each of R7, R8 and R9, which may be identical or different, denotes either a hydrogen atom or a halogen atom or an alkyl group containing from 1 to 20 carbon atoms, at least one of R7, R8 and R9 denoting an alkyl group. As an example of suitable compound there may be mentioned ethylaluminium dichloride or dibromide or dihydride, isobutylaluminium dichloride or dibromide or dihydride, diethylaluminium chloride or bromide or hydride, di-n-propylaluminium chloride or bromide or hydride, and diisobutylaluminium chloride or bromide or hydride. A trialkylaluminium such as tri-n-hexylaluminium, triisobutylaluminium, trimethylaluminium and triethylaluminium is employed in preference to the abovementioned compounds.
The cocatalyst may also be an aluminoxane. This aluminoxane may be linear, of formula
or cyclic, of formula
R denoting an alkyl radical containing from one to six carbon atoms and n being an integer ranging from 2 to 40, preferably from 10 to 20. The aluminoxane may include groups R of different nature. All the groups R preferably denote methyl groups. Furthermore, a cocatalyst is also intended to mean mixtures of the abovementioned compounds.
The quantities of cocatalyst which are employed must be sufficient to activate the transition metal. In general, when an organic aluminium derivative is employed as cocatalyst, a quantity thereof is introduced such that the atomic ratio of the aluminium contributed by the cocatalyst to the transition metal(s) which it is desired to activate ranges from 0.5 to 10 000 and preferably from 1 to 1000.
The polymerization or copolymerization process may be conducted in suspension, in solution, in gaseous phase or in bulk.
A bulk polymerization process consists in performing a polymerization in at least one of the olefins to be polymerized which is kept in the liquid or supercritical state.
The solution or suspension polymerization processes consist in performing a polymerization in solution or in suspension in an inert medium and especially in an aliphatic hydrocarbon.
In the case of a solution polymerization process it is possible to employ, for example, a hydrocarbon containing from eight to twelve carbon atoms or a mixture of these hydrocarbons. In the case of a suspension polymerization process it is possible to employ, for example, n-heptane, n-hexane, isohexane, isopentane or isobutane.
The operating conditions for these bulk, solution, suspension or gas-phase polymerization processes are those that are usually proposed for similar cases making use of conventional catalyst systems of the Ziegler-Natta type, whether supported or unsupported.
For example, in the case of a suspension or solution polymerization process it is possible to operate at temperatures ranging up to 250° C. and at pressures ranging from atmospheric pressure to 250 bars. In the case of a polymerization process in liquid propylene medium the temperatures may range up to the critical temperature and the pressures may be included between the atmospheric pressure and the critical pressure. In the case of a bulk polymerization process resulting in polyethylenes or in copolymers in which ethylene predominates it is possible to operate at temperatures of between 130° C. and 350° C. and at pressures ranging from 200 to 3500 bars.
A gas-phase polymerization process may be employed using any reactor which permits gas-phase polymerization, and in particular in an agitated-bed (e.g. stirred) and/or a fluidized-bed reactor.
The conditions for implementing the gas-phase polymerization, especially temperature, pressure, injection of the olefin or of the olefins into the reactor containing a stirred bed and/or a fluidized bed, and control of the polymerization temperature and pressure, are similar to those proposed in the prior art for the gas-phase polymerization of olefins. The operation is generally carried out at a temperature that is lower than the melting point Tm of the polymer prepolymer to be synthesized, and more particularly between +20° C. and (Tm−5)° C., and at a pressure such that the olefin or the olefins are essentially in the vapour phase.
The solution, suspension, bulk or gas-phase polymerization processes may involve a chain transfer agent, so as to control the melt index of the polymer to be produced. The chain transfer agent employed may be hydrogen, which is introduced in a quantity that can range up to 90% and preferably lies between 0.01 and 60 mol % of the combined olefin and hydrogen delivered to the reactor.
The olefins that can be employed for the polymerization or copolymerization are, for example, the olefins containing from two to twenty carbon atoms and in particular the alpha-olefins of this group. Olefins which may be mentioned are ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-octene, 1-hexene, 3-methyl-1-pentene, 3-methyl-1-butene, 1-decene, 1-tetradecene or mixtures thereof.
The polymerization or copolymerization process according to the invention is particularly suitable for reducing the heptane-solubles content of polymers or copolymers when the polymerization or copolymerization environment includes an olefin containing at least three carbon atoms. This process is therefore particularly suited for the polymerization or copolymerization of propylene.
In the examples which follow the heptane-insolubles content (represented by HI) was measured by extraction of the soluble fraction from the polymer using boiling heptane for two hours in an apparatus of the Kumagawa type.